The present invention relates to acoustic resonators, and more particularly, to acoustic resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has lead to a continuing need for small filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Accordingly, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter elements that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric material. In one simple configuration, a layer of piezoelectric material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the piezoelectric material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves can propagate longitudinally in the same direction as the electric field and reflect off the electrode/air interface. In addition, the sound waves also propagate in a direction transverse to the electric field and reflect off the various discontinuities at the edges of the electrodes or the structure.
The device is a mechanical resonator which can be electronically coupled. Hence, the device can act as a filter. For a given phase velocity of sound in the material, the mechanical resonant frequency is that for which the half-wavelength of the sound wave propagating longitudinally in the device is equal to the total thickness of the device. Since the velocity of sound is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and a few microns in thickness.
Thin film bulk acoustic resonators (FBARs) and stacked thin film bulk wave acoustic resonators (SBARs) include a thin sputtered piezoelectric film having a thickness on the order of one to two microns. Electrodes on top and bottom sandwich the piezoelectric film to provide an electric field through the piezoelectric material. The piezoelectric film, in turn, converts a fraction of the electric field into a mechanical field. An FBAR is a single layer of piezoelectric material and acts as an absorption filter. An SBAR is constructed by stacking two or more layers of piezoelectric material with electrodes between the layers and on the top and bottom of the stack. SBARs are typically used as transmission filters.
To simplify the following discussion, the present invention will be explained in terms of an FBAR; however, it will be apparent from the discussion that the teachings of the present invention are also applicable to SBARs as well. The portion of the piezoelectric film included between the overlap of electrodes forms an acoustic cavity. The primary oscillatory mode of this cavity is that in which sound waves, of the compression, shear, or plate wave type, propagate in a longitudinal direction perpendicular to the plane of the electrodes. Unfortunately, there are other oscillatory modes that can be excited. These so-called xe2x80x9clateral modexe2x80x9d resonances correspond to sound waves travelling parallel to the plane of the electrodes and bouncing off of the walls of the acoustic cavity or the discontinuity at the edge of the electrode layers. Once in these lateral modes, the mechanical energy is lost as heat. This loss of energy affects the quality of the FBAR. Reducing the energy loss from lateral mode resonances will improve the quality factor (Q) of the FBAR and permit the design of sharper frequency response filters, duplexers and oscillators with lower phase noise.
It is an object of the present invention to provide an improved, high Q bulk acoustic resonator with reduced energy loss from lateral mode resonances.
In accordance with one embodiment of the present invention, an acoustic resonator includes a substrate, first and second electrodes, and a piezoelectric material. The substrate has a depression formed in a top surface thereof. The first electrode, which is disposed over the depression in the top surface of the substrate, to provide an electrode/air interface, extends beyond the edges of the depression by a first distance to define a first region therebetween. The piezoelectric material is disposed on the top surface of the substrate and over the first electrode. The second electrode is disposed on the piezoelectric material and includes a portion that is located above the depression. The portion of the second electrode that is located above the depression has at least one edge that is offset from a corresponding edge of the depression by a second distance to define a second region therebetween. An overlap of the first and second electrodes and the piezoelectric material forms an acoustic cavity of the resonator. The first and second regions have impedances that differ from each other, as a result of the difference in materials in the two regions. In addition, each of the first and second distances is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the respective region, such that reflections off of the edges of the regions constructively interfere to maximize the reflectivity of the resonator. Thus, the first and second regions act as Bragg reflectors and reflect sound waves from lateral mode resonances back to the acoustic cavity of the resonator, where these sound waves may then be converted to the desired, primary oscillatory mode.
The acoustic resonator includes a further perimeter reflection system to reflect additional sound waves from lateral mode resonances back to the acoustic cavity of the resonator. The perimeter reflection system can include structures disposed on the piezoelectric material around the first electrode or structures disposed on the piezoelectric material above the depression and around the second electrode. An example of the former includes a structure located a predetermined distance from an edge of the first electrode corresponding to an edge of the second electrode that is offset from a corresponding edge of the depression. A third region extends from that edge of the first electrode to the structure. The third region has an impedance that differs from that of the second region, and the predetermined distance is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the third region. The structure itself defines a fourth region having an impedance different from that of the third region, and the width of the fourth region is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the fourth region.
A example of the latter structure is one disposed on the piezoelectric material above the depression and located a predetermined distance from an edge of the second electrode that is offset from a corresponding edge of the depression. A third region extends from that edge of the second electrode to the structure, and the predetermined distance is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the third region. Similar to the previous example, the structure itself defines a fourth region having an impedance different from that of the third region, and the width of the fourth region is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the fourth region.
In accordance with another embodiment of the invention, a method of making an acoustic resonator is described. The method includes providing a substrate having a depression formed in a top surface thereof and a first electrode disposed on the top surface. The first electrode is located above the depression and extends beyond the edges of the depression by a first distance to define a first region therebetween. The method further includes depositing a piezoelectric material on the top surface of the substrate over the first electrode and depositing a second electrode on the piezoelectric material. An overlap of the first and second electrodes and the piezoelectric material forms an acoustic cavity of the resonator. The second electrode includes a portion located above the depression that has at least one edge that is offset from a corresponding edge of the depression by a second distance to define a second region therebetween. The second region has an impedance that differs from that of the first region. In addition, each of the first and second distances is approximately equal to a quarter-wavelength of a sound wave travelling laterally across the respective region, and the first and second regions form Bragg reflectors. Additional structures, such as those described above, can also be added to reflect more sound waves from lateral mode resonances back to the acoustic cavity of the resonator.